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Nitrogen Rate Study Held True to Form

Applying 100 pounds of liquid 28% nitrogen sidedress on top of 20 pounds applied broadcast pre-plant, following soybean stubble, only produced 80 bushels per acre. Not good, but you've heard worse, right? How about this- 250 pounds of sidedress N on top of 20 plus bean stubble yielded a whopping 77 bushels per acre!

Wait a minute? Going backwards with more N? Taking more than three pounds of N to produce a bushel of corn? What gives? Stress, stress and more stress- just over 5 inches of rain all season long, with only one rain of an inch or more, some 28 'showers' in all, plus roughly 40 days of 90 degrees F or higher temperatures. To top it off, the loam soil turns into gravel at three feet, and the field wasn't irrigated.

The obvious question isn't why did the corn yield so poorly. Instead, it's: 'how did it yield as well as it did?' Compounding the stress this year was a planting rate of 32,000 seeds per acre. Why did the farmer plant so thick on the plots, located near Edinburgh, Ind.? Because for the past two years, rainfall was adequate, pushing ideal in '06, and yields were as high as 180 bushels per acre or more- on the same ground, and at the same N rates- 100 and 250 pounds per acre!

That's right- trends in rate vs. yield still held together fairly well, even under stress. Average yield for two hybrids topped out at 100 pounds of sidedressed N, then hovered between 78 and 77 bushels per acre at 150, 200 and 250 pounds per acre. Adding extra N didn't help in '06 either, in one of the best years ever in that location. Yield levels were higher, but the shape of the response curve was the same.

"We've put in the two ends of the spectrum," Dave Nanda said earlier. While a stressful, dry year was tough on yields and profits, it's valuable to a researcher. Nanda, a long-time corn breeder, is a consultant on the Corn Illustrated project. "It's valuable to know how your treatments react at opposite ends of the spectrum."

If you graph out this year's response for yield, it would look very similar to last year, Nanda notes, if not identical. The only addition this year was a 50 pound per acre rate. "We added it since yield topped out at 100 pounds per acre in '06," he notes. "So we wondered where the break was. We got a big chunk of that response with the first 50 pounds of sidedress N."

In fact, difference between 50 and 100 pound sidedress rates was 5.2 bushels per acre. At $4 corn and 43 cents per pound for N ($700 per ton for anhydrous), that's $21.50 cents expense for $20.80 return, a net loss! However, Nanda is not advocating stopping at 50 pounds per acre, even on droughty soils. Chlorophyll readings taken on both July 3 and July 25 indicated higher levels of chlorophyll, meaning more nitrogen to work with, at 100 pounds of sidedress N and up vs. 50 pounds, which in turn was considerably higher than readings at zero sidedress N.

Nanda argues that chlorophyll isn't a good indicator of yield from 100 pounds of N sidedress on up. Readings on the meter stagnated. But then, yield leveled out too. Purdue University corn specialist Bob Nielsen and soil fertility expert Jim Camberato are convinced they're seeing a correlation between chlorophyll readings and yield after studying it the past two seasons in various N trials within Indiana. Look for more results from the nitrogen trial in the February issue of your Farm Progress magazine.